Generally, as shown in FIG. 3, a side surface ground electrode 12 of a spark plug 11 is bent in a L-shape being perpendicular with respect to an axial core direction, so that a discharge part 14 facing an axial core electrode 13 is formed in a rectangular shape.
When a discharge spark occurs at the spark plug, a spark occurs between the axial core electrode 13 and a discharge end part 14 of the ground electrode 12 disposed below the axial core electrode 13. A mixed gas in a space 15 formed by the electrodes is combusted by spark, so that a compressed mixed gas is exploded in a cylinder. In the conventional art, a high pressure air generated by the explosion may be blocked by the discharge end part 14, so that a combustion spreading effect with respect to an air combustion mixed gas in a combustion chamber is not good.
When a residual carbon remains in the space formed by the electrodes, the carbons may be accumulated and be changed from a particle phase to a chain phase at the surfaces of the electrodes 13 and 14, so that a short circuit may occur between the electrodes 13 and 14. In this state, even when voltage is supplied, spark may not occur for thereby producing serious problems, so that an engine may stop or mixed gas is outputted to an exhaust pipe in a state it is not combusted. As non-combusted gas is outputted to an exhaust pipe, a backfire phenomenon frequently occurs, so that an abnormal phenomenon and a combustion efficiency decrease problem occur.
When a time of use passes, a crack may occur at a discharge concentration portion of the front end of the axial core electrode 13 due to corrosion, so that a critical damage occurs. The life span of the spark plug may decrease due to an unbalanced abrasion.
As shown in FIGS. 4 and 5, so as to overcome the above problems, there is provided a spark plug 26 which includes an axial core electrode 21 adapted to make spark, and a side surface ground electrode 25 which is opposite to the axial core electrode 21 with the side surface ground electrode 25 being disposed with a spark space 22 from the axial core electrode 21, and with a discharge end part 23 of the side surface ground electrode 25 being formed in a circular shape and having a center circular hole 24. A cap 27 of which part is opened is formed at the circular discharge end part 23 of the spark plug 26, and a plurality of protrusions 28 are formed at a circular inner side surface of the circular discharge front end part 23 in a circular shape. With the above construction, when a discharge spark occurs at the spark plug, a fast moving high pressure passes through the center circular hole 24 among the discharge end portion 23 of the ground electrode 25, the residual carbons are quickly sprayed from the space 22 between the electrodes 21 and 25, so that the residual carbons are not accumulated at a the discharge surfaces of the electrodes for thereby enhancing the efficiency of use and extending the life span of the products. In addition, the side surface ground electrode 25 is designed to effectively spreading heat energy occurring during a spark and explosion combustion from the circular discharge end part 23, so that the area of the spark generation may increase.
However, the spark plug having the circular discharge end part 23 is known to slightly prevent the problem that the carbons are accumulated on the surfaces of the electrodes. In addition, if the compressed mixed gas in the spark space 22 between the electrodes 21 and 25 is not uniformly distributed, the spark spreading speed and combustion efficiencies significantly decrease. The above problems occur because the sparks occurring at the spark plug do not affect the uniformly distributed mixed gas. The protrusions 28 formed at the inner side of the circular discharge end part 23 are formed with simple structures, so that the accumulated carbons can not be effectively removed from the surfaces of the electrodes because the sparks are spread in a straight line shape.